This invention relates to tensioners used with chain drives in automotive timing applications and more particularly to a dual tensioner system for a timing system that includes a balance shaft drive.
Tensioning devices are used as a control device for a power transmission chain as the chain travels between a plurality of sprockets. Generally, it is important to impart and maintain a certain degree of tension to the chain to prevent noises or slippage. Prevention of slippage is especially important in the case of a chain driven camshaft in an internal combustion engine because slippage may alter the camshaft timing by several degrees, possibly causing damage. In the harsh environment in which an internal combustion engine operates, chain tension can vary between excessively high or low levels as a result of the wide variations in temperature and differences between the coefficients of linear expansion among the various parts of the engine, including the chain and the tensioner. Camshaft and crankshaft induced torsional vibrations cause chain tension to vary considerably. This tension variation results in chain elongation. Moreover, wear of the chain components during prolonged use can cause elongation of the chain that results in a decrease in the tension of the chain.
In conventional engine timing systems, a tensioner arm is placed together with a hydraulic tensioner on the slack side of the timing chain and a chain guide is placed on the tight side. An example of such a conventional system is shown in FIG. 1. The engine has two separate banks of cylinders, each of which operates in a similar manner. Each engine bank has its own camshaft 4, 5, and camshaft sprockets 6, 7. The camshaft sprockets are connected to and driven by a crankshaft 2 through endless timing chains 8, 9 that are wrapped around the camshaft sprockets 6, 7 and the crankshaft sprockets 3, 3a.
The engine has a single crankshaft 2, but two crankshaft sprockets 3, 3a. One sprocket 3a is behind the other sprocket 3 in FIG. 1. One timing chain is wrapped around sprockets 3, 6 and the other timing chain is wrapped around sprockets 3a, 7. The arrow mark in FIG. 1 indicates the rotational direction of the crankshaft 2.
Tensioner arms 10, 20, which are designed to apply tension to the chains, and chain guides 30, 40, which are designed for regulating and controlling the running position of the timing chains, are placed on the slack side and tight side of the timing chains, respectively. The tensioner arms include a main body portion 11 and a shoe 12, which is made of rubber, resin or nylon. The shoes are mounted on the main body 11 on the chain contact side of each tensioner arm 10, 20. Bolt 13, which provides rotatable support for the arm body 11, is inserted in the hole formed in one end of the arm body 11. The bolt is fixed to the sidewall of the engine. The hydraulic tensioner 15 is placed on the opposite (free end) of the arm body 11 and its piston 15a presses the arm body toward the chain contact side.
Similar to the tensioner arms 10, 20, the chain guides 30, 40 are comprised of a main body portion 31, which is conventionally made of aluminum die cast, for example, and shoes 32 made of rubber, resin or nylon. The shoes 32 are mounted on the chain contact side of the guide body 31. The guide body is fixed to the engine sidewall by bolts 33 inserted in the holes at both ends.
FIG. 2 illustrates the use of tensioners and chain guides in another type of engine, specifically the timing system of an in-line dual overhead cam engine. In FIG. 2, the crankshaft 80 is attached to the drive sprocket 83 and the camshafts 81 and 82 are attached to the driven sprockets 84, 85, respectively. The timing chain 86 is wrapped around the driving sprocket 83 and the driven sprockets 84, 85. The tensioner arm 87 and the tensioner 88 are placed on the slack side of the timing chain 86 and the chain guide 89 is placed on the tight side of the timing chain 86. This tensioner arm 87, tensioner 88 and chain guide 89 are comprised in the same manner as tensioner arm 10, tensioner 15 and chain guide 40 of FIG. 1.
Chain guide 90 for guiding the timing chain is located between the driven sprockets 84, 85. Aperture 91 relates to providing lubrication of the chain and is not related to the present invention.
FIG. 3 illustrates the use of tensioners and chain guides in a conventional engine timing system that includes a balance shaft drive. In FIG. 3, the balance chain 100 is positioned around driven sprockets 97, 98 on the balance shafts 95 and 96, respectively, and the driving sprocket 99. Tensioner arm 101 and tensioner 102 are positioned on the slack side of the balance chain 100 and the chain guide 103 is positioned on the tight side of the balance chain 100. A chain guide 104, for guidance of the balance chain 100, is also placed between the driven sprockets 97, 98. Apertures 105, 106 in the chain guide 104 relate to discharge of oil from the tensioner and are not related to the present invention.
The present invention relates to a modification of the balance shaft drive system of FIG. 3. In such balance shaft drive systems, typically only a single tensioner, shown as tensioner 102 and arm 101, is used in the drive system. In such a system that has large crankshaft torsional loads and vibrations, however, a single tensioner is inadequate to provide proper tensioning of the system. Such torsional vibrations cause the balance shafts to rotate in clockwise and then counterclockwise directions (relative to the rotation of the system in the clockwise direction). One example of a system providing these high torsional loads is a twin cylinder engine. Another example of an application which would benefit from this system would be a multi-cylinder in-line diesel engine. The present invention is directed to providing a second tensioner in a balance shaft drive to maintain proper tension in the system. However, the present invention is not necessarily intended to be limited to a particular engine configuration or to balance shaft drives.